Not OP, but they're referring to situations when players call their own lines. (Virtually all amateur matches.) If you are horrible (or blatantly biased) at calling lines, your opponent can call the tournament referee, and you may end up with a line judge. But that takes time, and only happens after you aggravate your opponent. If you wait until a key moment to make a bad call in your favor, your opponent won't be able to reverse it, even if they call the referee and get a third party to call the lines for the rest of the match.

This is a particularly big issue in competitive junior tennis, unfortunately.